The present invention is directed to a bone screw of the type wherein a head of the bone screw is swingable or can swivel about the shank of the bone screw until the surgeon is satisfied with the relative placement of the two parts and thereafter the head can be locked in position relative to the shank.
Bone screws are utilized in many types of spinal surgery in order to secure various implants to vertebrae along the spinal column. Bone screws of this type typically have a shank that is threaded and adapted to be implanted into a vertebral body of a vertebrae. The bone screw also includes a head which is designed to extend beyond the vertebrae and which has a channel to receive another implant. Typically the channel will receive a rod or a rod-like member. In bone screws of this type, the head may be open, in which case a closure must be used to close between opposite sides of the head once a rod-like implant is placed therein, or closed wherein a rod-like implant is threaded through the head of a bone screw.
When the head and shank of the bone screw are fixed in position relative to each other, it is not always possible to insert a bone screw in such a manner that the head will be in the best position for receiving other implants. Consequently, swivel head bone screws have been designed that allow the head of the bone screw to rotate or swivel about an upper end of the shank of the bone screw while the surgeon is positioning other implants and finding the best position for the bone screw head. However, once the surgeon has determined that the head is in the best position, it is then necessary to lock or fix the head relative to the shank and different types of structures have been previously developed for this purpose. Unfortunately, the prior art devices have a tendency to be bulky, slip under high loading or require many parts.
It is desirable to have a swivel head bone screw that can be captured by the shank prior to locking of the head, but that allows the head to freely swivel or pivot about a top of the shank prior to locking. It is then further desirable to have the head that can be fixably locked in a configuration or position relative to the shank where the head best fits with other elements of the overall spinal implant.
Furthermore, it is desirable to maintain the number of parts of the device at a minimum. Also, it is desirable to secure the various parts together in such a way, so that, if parts become loose under use for some reason, the device will not totally disassemble.
A bone screw for use in conjunction with spinal surgery and, in particular, for implanting into a bone and securing other medical implants to the bone. The bone screw includes a head and a shank.
The shank has a lower portion which is threaded and sized and shaped to be operably screwed into a vertebral body in the spine of a patient. The end of the shank opposite the threaded lower portion includes a ball having a partial external thread thereon that is coaxial with an axis of rotation of the shank. The top of the shank includes a bore, preferably having a hexagonal cross-section or the like that is adaptable to a tool such as an alien wrench for driving the shank into the bone.
The head of the bone screw may be either an open or a closed type. Both types have a central channel that passes at least partially through the head. In the open type, the channel is open all the way to the top of the head and is closed after the placement of another implant into the channel by a closure. Where the head is closed, the channel that receives the implant is open on both ends, but encircled by the head.
The base of the head includes a chamber which is sized and shaped to fairly snugly receive the ball of the shank. In this manner the chamber and top of the shank function as a ball and socket arrangement that allows swiveling of the head relative to the shank until the head is locked relative to the shank. The chamber is connected to an exterior of the head by a threaded bore that is sized and shaped to threadably receive the ball from the shank with a minimum number of full rotations of the thread. That is, there are relatively few full 360xc2x0 turns of the thread on the ball such that the ball very quickly can be screwed through the bore and into the chamber. Once within the chamber, the thread on the ball becomes disengaged from the thread on the bore and the ball is able to freely rotate within the chamber, until locked.
At least one additional bore extends from an exterior of the head so as to intersect with the chamber. The second bore is aligned to intersect with the ball and receives a first set screw, when the ball is in the chamber or socket, that preferably engages the ball somewhat tangentially or at an offset angle with respect to the radius of the ball. The first set screw is also designed to engage the thread on the ball and to urge the ball against the sidewall of the chamber so as to lock the ball in position relative to the head. This in turn locks the shank in a fixed position or configuration relative to the head.
Preferably, a third bore and second set screw are located opposite the second bore and first set screw and the second set screw also operably engages the ball in a non-radial manner so as to likewise urge the ball against the wall of the chamber and to fix the ball in position relative to the head. Each set screw also preferably includes a tip which is partially deformable upon engaging the ball so as to help secure the ball in position and to enlarge and resist inappropriate disengagement of the set screw from the bore within which it is received. Also, each of the set screws preferably includes a breakaway installation head that is designed to break off at a preselected torque so as to set the set screws at a desired uniform torque with each installation.
Therefore, the objects of the present are: to provide a bone screw for implantation into a vertebra of a patient wherein the head of the bone screw is swingable or swivable about an end of a shank of the bone screw until a desired configuration is obtained after which the head is lockable in position relative to the shank; to provide such a bone screw wherein the shank includes a ball at one end thereof that is captured by the head, yet allows the head to remain swivable after capture; to provide such a bone screw wherein the shank ball is threaded exteriorly and wherein the head includes a chamber that is spaced from the exterior by a threaded bore that threadably receives the head and allows the head to pass from the exterior by screwing into the chamber after which the threads of the ball and bore disengage so that the ball is free to rotate and swing about within the chamber; to provide such a bone screw wherein a second bore is provided that extends from the exterior of the head to intersect with the chamber and receive a set screw therein that abuts against the ball when the set screw is advanced within the second bore; to provide such a set screw wherein the set screw engages the thread of the ball in a non-radial and somewhat tangential manner so as to urge the ball against the sidewall of the chamber and to lock the ball in position so that the shank is in turn locked in position relative to the head; to provide such a bone screw wherein the set screw includes a tip that expands upon engaging the ball to better resist relative movement of the ball and to also resist unplanned withdrawal of the set screw from the second bore due to vibration or the like; to provide such a bone screw swivel head which is readily adaptable for both open-headed and closed-headed bone screws; to provide such a bone screw wherein a single size head may be utilized in conjunction with various shank lengths such that fewer sizes of comparatively expensive bone screw heads are necessary to be kept in inventory; and to provide such a bone screw which is relatively easy to use and especially well suited for the intended usage thereof.
Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention.
The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.